42 METHODS OF CULTIVATION OF BACTERIA. 



rennet. The whey is strained off and the clot is hung up to 

 drain in a piece of musHn. The whey, which is somewhat 

 turbid and yellow, is then cautiously neutralised with 4 per cent 

 citric acid solution, neutral Htmus being used as an indicator. 

 When it gives a good neutral violet colour with the litmus, it is 

 heated upon a water bath at 100° C. for half an hour or so ; 

 thereby nearly the whole of the proteid is coagulated. It is 

 then filtered clear, and neutral Utmus is added to a convenient 

 colour for titrations or rougher observation." It may be steril- 

 ised in tubes or in bulk at 100° C, or by passing through a 

 Berkfeld filter. 



Use of neutral red. — This dye has recently been introduced 

 as an aid in determining the presence or absence of members 

 of the B. coli group, especially in the examination of water. 

 The medium found most suitable is agar containing .5 per cent 

 of glucose, to which i to 5 per cent of a concentrated solution of 

 neutral red is added. The use of this medium and its probable 

 value are described below. 



Blood Agar and Serum Agar. — The former medium was 

 introduced by Pfeiffer for growing the influenza bacillus, and it 

 has been used for the organisms which are not easily grown on 

 the ordinary media, e.g. the gonococcus and the pneumococcus. 

 Human blood or the blood of animals may be used. " Sloped 

 tubes " (vide p. 50) of agar are employed (glycerin agar is not so 

 suitable). Purify a finger first with i-iooo corrosive sublimate, 

 dry, and then wash with absolute alcohol to remove the sublimate. 

 Allow the alcohol to evaporate. Prick with a needle sterilised 

 by heat, and, catching a drop of blood in the loop of a sterile 

 platinum wire (vide p. 52), smear it on the surface of the agar. 

 The excess of the blood runs down and leaves a film on the 

 surface. Cover the tubes with india-rubber caps, and incubate 

 them for one to two days at 37" C. before use, to make certain 

 that they are sterile. Agar poured out in a thin layer in a 

 Petri dish may be smeared with blood in the same way and 

 used for cultures. In investigating the diseases of races other 

 than the white, it appears advisable to use the blood of the race 

 under investigation. 



Serum agar is prepared in a similar way by smearing the sur- 

 face of the agar with blood serum, or by adding a few drops of 

 serum to the tube and then allowing it to flow over the surface. 



